Mixtures of polymers have been previously described which are said to be miscible within certain temperature ranges and/or in certain polymer ratios. However, the overwhelming majority of polymers employed in mixtures are immiscible in each other. Furthermore, unless there is direct evidence showing miscibility, all mixtures of polymers should be presumed to be immiscible. Relevant reviews of the subject of polymer miscibility include S. Krause, Journal of Macromolecular Science, Reviews of Macromolecular Chemistry, C7, 251 (1972); D. R. Paul (ed.) "Polymer Blends", Volume 1, pp. 15-113, Academic Press, New York, 1978; and O. Olabisi, L. M. Robeson and M. T. Shaw, "Polymer-polymer Miscibility", pp. 215-276, Academic Press, New York, 1979.
Polymer mixtures have also been described in various other publications. For example, U.S. Pat. No. 3,949,021 discloses solutions of water-insoluble polymers and cross-linking agents in monomeric N-vinylpyrrolidone which are polymerized to provide hard, solid materials which may be machined, cut and/or polished. These materials are crosslinked hydrogels which are said to be suitable for extended-wear contact lenses. However, the crosslinked nature of such materials causes them to lose their thermoplasticity.
U.S. Pat. No. 4,300,820 discloses compositions of N-vinylpyrrolidone with water-insoluble copolymers. Ethylenically unsaturated monomers which contain acid groups are included in the coompositions. However, such compositions do not form miscible blends as that term is defined herein.
U.S. Pat. No. 3,287,411 discloses mixtures of a poly(N-vinyl amide) and a polymer from the group of polyamides, polyureas and polyurethanes. The mixtures are said to be homogeneous.
U.S. Pat. No. 3,798,289 discloses that mixtures of polyvinyl chloride and a copolymer of N,N-dimethylacrylamide and ethylene are miscible at a 1:1 ratio. However, such mixtures are incapable of absorbing sufficient water to provide compositions containing more than 50% by weight water due to the high level of hydrophobic polyvinyl chloride in the composition.
Other mixtures of polymers are disclosed in J. Appl. Polymer Science, 9, 1385 (1965). These mixtures include polyethylene oxide and polyvinylnaphthalene. This mixture is compatible only when more than 45 weight % polyvinylnaphthalene is present. However, such a high level of a hydrophobic material renders the mixture incapable of providing compositions having more than 50% by weight water.
U.S. Pat. No. 4,279,794 discloses graft copolymers wherein hydrophobic branches are covalently grafted to hydrophilic backbones. These polymers form immiscible "seas" and "islands". The seas typically comprise the hydrophilic phase while the islands typically comprise the hydrophobic phase. See S. Yamashita, K. Shibatani, K. Takakura and K. Imai, "Reinforced Biocompatible Hydrogel", Kobunshi Ronbunshu, 39(4), pg. 187-95.
The J. Colloid Interface Sci. 31,168 (1969) discloses a miscible blend of polyethylene oxide and polypropylene oxide. Such blends are miscible only for low molecular weight polymers in the liquid phase.
Nippon Setchaku Kyokai Shi, 11, 2(1975) discloses that copolymers of N-vinylpyrrolidone and vinyl acetate (PVNP/VA) can be mixed with other polymers to provide compositions that contain 50 weight % PVNP/VA and 50 weight % other polymer. Such blends are incapable of providing compositions containing more than 50% by weight water due to the high level of hydrophobic component.
Polymer Prep., Amer. Chem. Soc. Div. Polymer Chem. 10, 385 (1969) discloses mixtures of polyvinyl acetate and a copolymer of polymethyl vinyl ether and maleic anhydride.